The word natural here is meant in contrast to it being a sociological construct.

Is kissing in all its forms something natural for humans? Is it instinctively erotic? Or is it just a conventional form to show trust and intimicy, i.e. the association besically just comes via a social means?

Because the only other advantage of this mouth to mouth contact I could see is maybe for the immune system.

It's probably one of the arbitrary reward systems for actions beneficial to reproduction. Social contact is advantageous for reproduction, so rewarding it makes sense. Kissing may just have developed because the lips are full of sensory receptors for food-proofing purposes already; so it's not a big effort to use them for additional purposes.
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ArmatusJun 4 '12 at 11:25

1 Answer
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Wikipedia actually has a note on the evolution of kissing. They point out that there are grooming behaviors such as licking and social behaviors like dogs touching noses in many species even insects. They note that it is not decided at all whether kissing is learned or from some sort of instinct. They also mention that its reminiscent of feeding behavior common in birds (and other animals) where food is pushed down the throats of young. (try thinking about that next time!)

I seem to remember though that there are some culture which do not kiss. with their lips at least. This would favor kissing as a learned behavior, but these cases are very rare and isolated peoples and it might be simply a taboo to talk about kissing with researchers or at all, which is a hazard of that sort of work. In addition, bonobos, one our our closest primate cousins, do kiss frequently with the lips, as a social and intimate gesture and the parallels with most human cultures are difficult to ignore.